The present invention relates to steam turbines and, more particularly, to control systems for steam turbines. Even more particularly, the present invention relates to means for reducing the possibility of turbine damage resulting from water carryover from the boiler into the steam turbine.
Steam turbines are designed to operate with a supply of steam produced by a boiler. During certain phases of operation, there is a risk that water may enter the steam turbine along with the steam. Water carryover is undesirable since the resulting mechanical vibrations and thermal shock may result in shortened lifetime or immediate permanent damage to the steam turbine.
Water carryover may be produced, for example, by an excessive steam demand compared to the existing boiler generating capacity or the complement of this condition, an insufficient steam supply compared to the steam demand. Although these two conditions may be considered to be similar, they can be roughly differentiated according to their causes. Excessive steam demand can occur, for example, during startup when the rate of increase in the steam fed to the steam turbine exceeds the rate of increase in steam generation of the boiler. This mismatch results from the fact that, whereas a steam turbine is capable of responding quite rapidly to additional steam fed to it, the boiler, depending as it does on the less rapidly responding heat source, may not be able to increase its output at a corresponding rate. This is especially true of coal-fired boilers. An insufficient steam supply may occur, for example, due to a fire outage in the boiler, a problem in the boiler controller or an improperly chosen boiler setpoint.
During startup of a steam turbine driving an electric generator, for example, the steam turbine is manually or automatically brought up to running speed under no load or, at most, under light load. The electric generator is then energized and the torque output of the steam turbine is gradually increased by automatically or manually increasing the flow of steam fed to it. Ideally, the steam being generated should increase at the same rate as the steam flow. As noted above, however, and particularly in the case of a coal-fired boiler, the responses of the steam turbine and the boiler cannot be perfectly matched. Thus, during startup at least, the boiler pressure must be permitted to vary within limits. When the boiler pressure falls too low too rapidly, however, water carryover can occur.
One method for avoiding water carryover provides for manually or automatically reducing, or even cutting off, steam flow when the steam pressure decreases to a value lower than a fixed or variable threshold. The thresholding method may not be adequate during startup when the rate of change in boiler pressure can exceed the response capability of the steam valve control system. If the steam valve control system is additionally made responsive to the rate of change in steam pressure, a way is provided for predicting an impending condition of water carryover early enough for timely control of steam flow by the steam valve control system.
Although steam pressure rate control is attractive, certain problems require solutions. As is well known, steam turbines, particularly in electric utility service, remain in operation for uninterrupted times measured in months to years. In addition, although rapid pressure variations may occur at times, very slow pressure changes may also require tracking over such extended periods. Control devices for such service must therefore be stable and drift-free. Rate measuring devices of the prior art suffer from drift which makes them marginal at best for this use.